The Origins of the Latitudinal Diversity Gradient: Revisiting the Tropical Conservatism Hypothesis

Aim: Understanding the origins of species richness patterns (especially high tropical richness) is a long-standing challenge at the intersection of biogeography, ecology, and evolutionary biology. One hypothesis that can potentially explain the latitudinal richness gradient is the tropical conservatism hypothesis (TCH). The TCH proposes that there are presently more species in tropical regions because many clades originated in the tropics and have only colonised the temperate zones more recently, leaving less time for speciation to build up temperate richness, and with niche conservatism limiting temperate colonisation by tropical clades. Here, we review the empirical evidence for the TCH. We first define this hypothesis, outline its major predictions, and describe its relationship to similar hypotheses. We then perform a systematic review to quantitatively evaluate the support for (and against) its major predictions. Finally, we describe several areas for future research. Location: Global. Time Period: Present to ~750 million years ago. Major Taxa Studied: All (especially plants and animals). Methods: We perform a systematic review of the evidence for the TCH over the last ~20 years. Results: Most predictions of the TCH were supported in a significant majority of the studies that examined them. Further, a significant majority of relevant studies rejected the role of higher tropical diversification rates in driving the latitudinal diversity gradient (contrary to the diversification-rate and out-of-the-tropics hypotheses). Surprisingly, the importance of diversification rates did not depend on the ages of the clades studied. Main Conclusions: Our results generally support the TCH, but also highlight several important issues moving forward. Most studies tested very few predictions of the TCH, and the pivotal role of colonisation time was often untested. Many studies analysed phylogenetic diversity measures, but their relevance for explaining richness patterns remains disturbingly unclear. Finally, we discuss several unresolved questions about the TCH and the origins of richness patterns.